You just specify the folder number in the request.
Code:
<CreateAppointmentRequest
[echo="1"] // if specified, the created appointment is echoed back in the response as if a GetMsgRequest was made
[max="{max-inlined-length}"] [html="*0|1"] [neuter="0|*1"] // options for echoing
>
<m f="{flags}" l="{folder}"> // See soap.txt for complete list of attrs on <m>
[<e .../>*] // users to send request to
<su>{subject}</su> l="{folder}" specifies the folder you want the appointment created in